A mobile host MH and its home network HN correspond to both the Internet Protocol version 4 (IPv4) and the Internet Protocol version 6 (IPv6). An authentication server (RS) is accommodated in the home network HN. A corresponding host H corresponds to the IPv6 and is accommodated in the IPv6 network. In addition, a plurality of relay servers S1, S2 and S3 corresponding to both the IPv4 and the IPv6 are accommodated in the IPv6 network. The respective S1, S2 and S3 extract an IPv6 packet by decapsulating the IPv4 packet received from the mobile host MH via the IPv4 network, and transfers the relevant IPv6 packet to the corresponding host H via the IPv6 network.